A pump apparatus comprising a pump piston reciprocably driven within a cylinder to effect pumping of crude oil experiences considerable wear, particularly at the seal between the piston and the cylinder wall, and in the check valves which control the flow of the crude oil into and out of the pump chamber, because of sand and other particulate material carried along with the oil. Even where the pump apparatus is part of an undersea installation, it is not usually difficult to effect replacement of the check valves when needed, but replacement of the piston seal and/or the piston itself present more serious difficulty and may require disassembly of the apparatus.